Fuel control for gas turbine engine

ABSTRACT

Fuel control apparatus including a fuel conduit connected to receive pressurized fuel from a fuel pump and provided with fuel servo actuated metering valve means therein for controlling the rate of fuel flow therethrough to an engine. The fuel servo actuated metering valve is actuated by a fuel servo piston pressurized by a servo valve controlled fuel pressure differential derived from a predetermined fuel pressure differential maintained by a pressurizing valve downstream from and in series flow relationship with the metering valve. The metered fuel flow for actuating the servo piston is extracted from the fuel conduit upstream from the pressurizing valve and returned to the fuel conduit downstream from the pressurizing valve.

BACKGROUND OF THE INVENTION

Conventional fuel control systems utilizing fuel operated servo devicesfor actuating a positionable member such as a fuel metering valve orother engine control devices are well known in the art. It has been acommon practice to extract from fuel pump output flow the requiredpressurized fuel flow for input to the servo and vent servo exhaust flowto a relatively low pressure fuel source such as the inlet of the fuelpump. Obviously such an arrangement wherein relatively high pressurefuel discharged by the fuel pump and used for servo control purposesundergoes a substantially large pressure drop in passage to the fuelpump inlet and resultant energy loss. Furthermore, the fuel pumpcapacity which generally dictates size and weight of the fuel pump mustbe selected to fulfill the fuel flow requirements of the engine. Inaddition, the increased fuel pump capacity necessitated by the servomeans places a corresponding higher demand on power input to drive thefuel pump.

Reference is made to U.S. Pat. No. 3,521,447 to F. R. Rogers and D. L.Greenawalt for example of a known fuel control having a fuel operatedservo network wherein the total output flow of a fuel pump is availablefor energizing fuel operated servo means as well as for fuel meteringpurposes to operate an engine. The subject matter of U.S. Pat. No.3,521,447 represents a significant advance in the fuel control art inthat it avoids the above-mentioned undesirable features normally foundin fuel controls embodying fuel operated servo networks. The presentinvention represents an improvement over the apparatus shown anddescribed in U.S. Pat. No. 3,521,447 in that the same desired resultsare obtained in a much simpler manner with less complicated structure.

SUMMARY OF THE INVENTION

The present invention relates to a gas turbine engine fuel controlhaving a fuel operated servo actuated fuel metering valve which servoincludes a piston responsive to pressurized fuel diverted from a mainmetered flow of fuel established by the metering valve and subsequentlyreturned to the main metered flow of fuel.

It is an object of the present invention to provide a fuel controlhaving metering valve means for establishing a metered flow of fuel andfuel operated servo apparatus for actuating the metering valve inresponse to a fuel pressure differential derived from the flow ofmetered fuel.

It is another object of the present invention to provide a fuel controlhaving metering valve means for establishing a metered flow of fuel andfuel operated servo apparatus for actuating the metering valve inresponse to a fuel pressure differential derived from a fuelpressurizing valve in series flow with and downstream of the meteringvalve.

Other objects and advantages of the present invention will be apparentto those skilled in the art from the following description taken withthe accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic representation of a gas turbine engine andassociated fuel control apparatus embodying the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing, numeral 10 designates a conventional gasturbine engine having an air inlet 12, an air compressor 14, a pluralityof combustion chambers 16, a gas turbine 18 connected via a shaft 20 tothe compressor 14 to drive the same, and an exhaust nozzle 22 from whichthe products of combustion are expelled to the atmosphere. A pluralityof fuel injection nozzles 24 connected to a fuel manifold 26 are adaptedto inject metered pressurized fuel into the combustion chambers 16 wherethe resulting air fuel mixture is burned to generate hot motive gaswhich passes through turbine 18 to drive compressor 14 and exhauststhrough nozzle 22 to the atmosphere to generate a propelling thrust.

A metered flow of fuel is supplied to fuel manifold 26 via a fuel tank28, an engine driven fuel pump 30 of the positive displacement type, afuel control generally indicated by 32 and including a hydromechanicalfuel flow control section 34 and electronic sensing and signal computingsection 36.

The electronic sensing and signal computing section 36 is conventionalstructurally and operationally in that it is connected to receiveelectrical input signals representing selected variable conditions ofengine operation as, for example, engine speed N, compressor dischargeair pressure P_(c), power lever position PLA and compressor inlet airtemperature T_(i) or other engine temperatures. The electrical inputsignals are sensed, compared electronically in a conventional mannerresulting in a computed electrical signal which may be suitablyamplified and discharged as an electrical output signal to the fuelsection 34 for control purposes as will be described.

The hydromechanical fuel flow control section 34 includes a casing 38having a fuel inlet 40 connected to receive pressurized fuel P₁ fromfuel pump 30 via a conduit 42 and a fuel outlet 44 connected to suppliedmetered fuel flow to fuel manifold 26 via conduit 46. The inlet 40 andoutlet 44 are connected by a main fuel conduit which includes a passage48, a fuel metering valve restriction 50, a passage 52, a fuelpressurizing valve restriction 54, a passage 56, a fuel cutoff valve 58and a passage 60. A fuel metering valve 62 slidably carried by casing 38is provided with a tapered end 64 which cooperates with restriction 50to vary the effective flow area thereof. The opposite end of meteringvalve 62 is fixedly secured to a servo piston 66 slidably carried in achamber 68 which is vented to passage 52 intermediate restrictions 50and 54 via a passage 70 containing a restriction 72. A passage 74containing a restriction 76 communicates chamber 68 with passage 56intermediate restriction 54 and cutoff valve 58. The passages 70 and 74are connected via a passage 78 containing a restriction 80. Therestrictions 72, 76 and 80 are preferably of the same effective flowarea. The passage 70 downstream from restriction 72 is vented to passage56 intermediate restriction 54 and cutoff valve 58 via a passage 82,servo valve restriction 84, a chamber 86 and a passage 88. The effectiveflow area of restriction 84 and thus servo fuel pressure P_(s) inchamber 68 is controlled by a movable servo valve 90 actuated by aconventional proportional electrical solenoid generally indicated by 92which is electrically energized by electronic computing section 36 viasuitable electrical leads 94, 95 and 96. The solenoid 92 is removablysecured to casing 38 by suitable fastening means such as bolts 97.

The piston 66 is responsive to fuel pressure differential P_(s) -P₃established thereacross in chamber 68 via passages 70 and 74. Maximumand minimum open positions of metering valve 62 are established byadjustable stop means including a screw member 98 threadedly engagedwith casing 38 and provided with a threaded stem 99 to which a nut 100is threadedly secured. An arm 102 extending from piston 66 is slotted asat 104 to slidably receive stem 99 thereby allowing arm 102 to engagenut 100 or screw member 98 and limit the travel of piston 66 andmetering valve 62 accordingly. It will be noted that screw member 98 maybe adjusted to establish the maximum open position and thus maximum flowposition of metering valve 62 whereas nut 100 may be adjusted toestablish the minimum open position and thus minimum flow position ofmetering valve 62.

A predetermined constant fuel pressure differential is maintained acrossmetering valve restriction 50 regardless of the position of meteringvalve 62 by a bypass valve 106 disposed in a fuel bypass conduit 108connecting passage 48 with a fuel return passage 110 leading to theinlet of pump 30 at relatively low pump inlet pressure P_(o). The bypassvalve 106 is slidably carried by casing 38 and actuated by a diaphragm112 fixedly secured to one end of valve 16. The diaphragm 112 is fixedlysecured at its outermost peripheral portion to casing 38 and partiallydefines two chambers 114 and 116. Chamber 114 is vented to passage 48 atfuel pressure P₁ via radial and axial passages 118 and 120,respectively, in valve 106. Chamber 116 is vented to passage 52 atmetered fuel pressure P₂ via a passage 122 containing a restriction 124.A compression spring 126 interposed between casing 38 and by pass valve106 serves to preload the valve 106 in a closing direction against theopposing force derived from diaphragm 112 to thereby regulate bypassflow through conduit 108 and thus pressure P₁ as necessary to maintainthe P₁ -P₂ differential at the selected value.

A fuel pressure relief valve 128 loaded by a compression spring 130vents passage 48 to passage 108 in the event fuel pressure P₁ reaches apredetermined maximum allowable value.

The fuel cutoff valve 58 operates to shut off fuel flow to the fuelmanifold and simultaneously vent passage 56 at metered fuel pressure P₂to relatively low fuel pump inlet pressure P_(o). To that end, thecutoff valve 58 is slidably carried by casing 38 and provided withspaced apart lands 132 and 134 which move into or out of engagement withpassages 56 and 60, respectively, in response to movement of a powercontrol lever 136. The power control lever 136 is fixedly secured to ashaft 138 suitably mounted for rotation in casing 38 and provided with alever arm 140. The cutoff valve 58 has a slotted end portion 142 adaptedto slidably receive the free end 44 of lever arm 140. The end portion142 is exposed to a chamber 146 vented via a passage 148 to bypassconduit 108 at pump inlet pressure P_(o). A notch or port 150 formed inland 134 is adapted to clear casing 38 thereby venting passage 60 tochamber 146 at pressure P_(o) when valve 58 is actuated to a fuel cutoffposition.

The effective flow area of fuel pressurizing valve restriction 54 andthus pressure differential P₂ -P₃ thereacross is controlled by acup-shaped pressurizing valve 152 slidably carried in casing 38 and, inpart, defining a chamber 154 which chamber 154 is vented via a passage155 to passage 56 at fuel pressure P₃. A compression spring 156interposed between valve 152 and a spring retaining cap or plug 158removably secured to casing 38 by any suitable fastening means such asbolts 160 serves to preload valve 152 in a closing direction inopposition to the force derived from fuel pressure differential P₂ -P₃acting across the base of valve 152. The use of a spring loadedpressurizing valve to establish a predetermined fuel back pressure in afuel control for control purposes is a common feature in known fuelcontrols. However, it will be noted that Applicant's pressurizing valve152 serves a dual purpose in that it not only establishes a fuel backpressure by remaining closed until spring 156 is overcome by apredetermined fuel pressure P₂ but also, upon valve 152 opening, thechamber 154 is pressurized by the fuel pressure P₃ in passage 56. Thethrottling effect of valve 152 on metered fuel flow through restriction54 serves to maintain the pressure differential P₂ -P₃ at a constantvalue depending upon the load exerted by spring 156.

An electrical signal representing compressor discharge pressure P_(c) isproduced by an evacuated bellows 162 exposed to a chamber 164 which isvented to the discharge end of compressor 14 via passages 166 and 168.The bellows 162 is anchored at one end to casing 38 and at its oppositemovable end, is fixedly secured to a spring retaining plate 170. A stemor rod 172 fixedly secured to plate 170 is provided with a circular slug174 of magnetic material fixedly secured thereto. A cup-shaped cap 176removably secured to casing by suitable fastening means such as screws178 is provided with an annular section 180 which is adapted to receiveslug 174 in radical spaced relationship therewith and which is providedwith wire coils 182 embedded therein. Electrical leads 184 and 186connect coil 182 to electronic section 36 to thereby transmit anelectrical signal indicative of the position of bellows 162 in responseto compressor discharge pressure P_(c).

A metering valve 62 position feedback electrical signal is generated bya circular slug 188 fixedly secured to a stem 190 which stem 190 isfixedly secured to valve 62 and axially movable therewith. The slug 188is received by an annular section 192 of a cup-shaped cap 194 fixedlysecured to casing 38 by suitable fastening means such as screws 196 and,like cap 176, provided with wire coils 198 embedded therein. Electricalleads 200 and 202 connect coils 200 to electronic section 36 to therebytransmit an electrical signal indicative of the position of meteringvalve 62.

An electrical signal representing power lever 136 position is generatedby a conventional potentiometer 204 having a rotatable shaft 206 onwhich a gear 208 is mounted to actuate the same. A gear 210 fixedlysecured to shaft 138 is meshed with gear 208 to drive gear 208 inresponse to movement of power lever 136. Electrical leads 212 and 214connect potentiometer 204 to electronic section 36 to thereby transmitthe electrical signal therebetween.

Emergency control of fuel flow in the event of electrical power failureis provided by normally closed conduit means in parallel flowrelationship with metering valve 62 and pressurizing valve 152, whichconduit means includes a passage 216 connected to passage 48 atunmetered fuel pressure P₁, a movable valve member 218 actuated to anormally closed position by an electrically energized solenoid 220provided with electrical leads 222 and 224 to electronic section 36, apassage 226 terminating in a variable area restriction 228 controlled bya positionable valve 230, a chamber 232 and a passage 234 leading fromchamber 232 to passage 56 downstream from pressurizing valve 152. Abellcrank pivotally secured to a fixed support 236 is provided with anarm 238 having a positionable valve 230 fixedly secured thereto and asecond arm 240 engageable with a cam follower pin 242. The cam followerpin 242 is slidably carried in an opening 244 in casing 38 whichseparates chamber 232 from a chamber 246. The follower pin 242 bearsagainst a rotatably and axially movable three dimensional cam 248secured to a shaft 250 extending from one end thereof into a recess 252in casing 38 which slidably supports shaft 250 for rotatable and axialmovement. The opposite end of cam 248 is fixedly secured to a free endof an evacuated bellows 254 coaxial with cam 248 and exposed to chamber246. The opposite end of bellows 254 is anchored to one end of arotatable shaft 256 which extends through an opening 258 in casing 38into engagement with manually actuated power lever 260 fixedly securedthereto. The shaft 256 is prevented from shifting axially by spacedapart flanged portion 262 and washer 264 which slidably engage casing38. A snap ring 266 engageable with shaft 256 retains washer 264 inposition on shaft 256. The chamber 246 is vented to ambient oratmospheric air pressure P_(A) via a port 268. It will be understoodthat the cam 248 is rotatably positioned by power lever 260 and axiallypositioned by bellows 254 which expands or contracts, depending upon therelative change in pressure P_(A) imposed thereon.

The engine 10 is put in operation by actuating the power lever 136 to anengine start position which through conventional control apparatus, notshown, acts to crank the engine 10 to a self-sustaining engine speed aswill be recognized by those persons skilled in the gas turbine enginefield. During engine cranking the fuel pump 30 is driven by the engineresulting in a flow of pressurized fuel to inlet 40 and thuspressurization of the interior of casing 38. The fuel pressurizing valve152 is held in a closed position by spring 156 until a predeterminedfuel pressure P₂ in passage 52 is reached whereupon valve 152 opens toadmit fuel to passage 56. Assuming the powder lever 136 is advanced toan engine idle position, the cut-off valve 58 will occupy an openposition permitting fuel to pass through outlet 44 and conduit 46 tofuel manifold 26 thereby pressurizing injectors 24. The fuel pressure P₃in passage 56 is transmitted through passage 155 to chamber 154 where itacts against valve 152 in opposition to fuel pressure P₂ causing closingmovement of valve 152 to increase the pressure differential P₂ -P₃thereacross which, upon reaching a predetermined value and incombination with the effective area of valve 152 exposed thereto,produces a force balancing the opposing force of spring 156. The valve152 will respond to a change in pressure P₂ or P₃ tending to upset thebalanced condition of valve 152 and actuate valve 152 in a closed oropen position as necessary to maintain the predetermined differential P₂-P₃ thereacross.

The potentiometer 204 is positioned by power lever 136 therebygenerating a corresponding output signal to electronic section 36 whichsection 36 also receives an input electrical signal representingexisting engine speed N as pointed out heretofore. The power request andengine speed signals are compared resulting in a speed error signalwhich is transmitted to solenoid 92 which, in turn, is energized causingvalve 90 to move in a closing direction in proportion to the errorsignal. The resulting increase in pressure P_(s) unbalances piston 66 ina direction to open metering valve 62 thereby increasing fuel flowthrough metering restriction 50 causing an increase in pressure P₂ inpassage 52 and thus a decrease in pressure differential P₁ -P₂ acrossmetering restriction 50 as well as an increase in pressure differentialP₂ -P₃ across pressurizing valve 152. The slug 188 moves axially throughannular section 178 in response to movement of valve 62 therebygenerating an electrical position feedback signal to electronic section36 which feedback signal is compared to the output signal applied tosolenoid 92 to ensure the desired position of valve 62. The bypass valve106 is actuated toward a closed position by diaphragm 112 in response tothe increase in pressure P₂ thereby reducing the bypass flow to increasepressure P₁ as necessary to reestablish the predetermined differentialP₁ -P₂. The pressurizing valve 152 is actuated in an opening directionin response to the pressure P₂ increase thereby reducing the flowthrottling effect of valve 152 which results in an increase in pressureP₃ in passage 56 to the extent that the predetermined differential P₂-P₃ is reestablished across pressurizing valve 152. It will beunderstood that the abovementioned pressure variations are relativelybrief, such that the pressure differentials P₁ -P₂ and P₂ -P₃ may beconsidered substantially constant regardless of the area change ofmetering restriction 50. The increase in fuel flow passes through cutoffvalve 58 and outlet 44 to the engine 10 causing an increase in speed Nwhich, in turn, is transmitted to electronic section 36. As speed Nincreases toward the desired idle condition the speed error outputsignal applied to solenoid 92 diminishes and ultimately becomes zeroupon engine speed stabilizing at the selected idle speed. Movement ofpower lever 136 to higher power positions requesting an engineacceleration results in a similar sequence of operation whereas movementof power lever 136 from a higher to a lower power position requestingengine deceleration results in a reverse sequence of operation as willbe understood by those persons skilled in the art.

Preferably the restrictions 72, 76 and 80 are selected with equaleffective flow areas such that the P₂ -P₃ ¹ pressure drop acrossrestriction 80 is equal to the drop P₃ ¹ -P₃ across restriction 76 orone-half the total pressure drop P₂ -P₃ across both restrictions 80 and76. The pressure P₃ ¹ intermediate restrictions 80 and 76 and actingagainst piston 66 is therefore maintained midway between pressures P₂and P₃ such that a relatively large pressure differential P_(s) -P₃ ¹ isavailable at all times to power the piston 66 in the event of stickingof the latter.

In the event of an electrical power failure, the solenoid 92 isdeenergized and suitably designed to move valve 90 to a fully openposition thereby reducing pressure P_(s) which unbalances piston 66 andthus metering valve 62 to a minimum flow position. Simultaneously, thepower failure causes deenergization of solenoid 220 which moves valve218 to a fully open position whereupon fuel flow through passage 226 iscontrolled by the effective area of valve 230. The position of valve 230is dependent upon the position of cam 248 which is rotated by powerlever 260 and actuated axially in response to atmospheric or compressorinlet air pressure P_(A) thereby establishing an emergency fuel flowthrough passage 234 which is a function of power lever 260 positionmodified by atmospheric pressure P_(A).

It wll be noted that the pressurizing valve 152 provides a relativelylarge fuel pressure differential P₂ -P₃ for powering the metering valvepiston 66 and eliminates the need for conventional separate fuelpressurizing valve apparatus in the main supply conduit to the engineand fuel pressure regulating valve apparatus for servo use. In addition,the metering valve 62 stroke may be made relatively large to reduce gainsensitivity of the metering valve 62 thereby improving accuracy.

I claim:
 1. Fuel control apparatus for a combustion engine comprising:afuel supply conduit for conducting a flow of pressurized fuel from apressurized source to said engine; first positionable valve meansoperatively connected to said supply conduit for establishing acontrolled flow of fuel therethrough to said engine; first fuel pressuredifferential responsive means operatively connected to said first valvemeans for actuating the same; second valve means operatively connectedto said supply conduit in series flow relationship with said first valvemeans and downstream therefrom for creating a predetermined backpressure against fuel flow through said first valve means .[.and.]..Iadd.by .Iaddend.restricting fuel flow .Iadd.in the supply conduit.Iaddend.to establish a predetermined fuel pressure differential .[.insaid supply conduit.]..Iadd., said second valve having a wall that movesin an opened direction in response to said predetermined fuel pressuredifferential to allow fuel to flow to said engine; .Iaddend. resilientmeans operatively connected to said second valve means for urging thesame toward a closed position in opposition to the fuel pressureupstream therefrom .[.second fuel pressure differential responsive meansvented to said supply conduit upstream and downstream from said secondvalve means and responsive to the fuel pressure differentialtherebetween for actuating said second valve means in an openingdirection in opposition to said resilient means;.]. .Iadd.to establish aminimum fuel pressure differential; .Iaddend. first passage meansconnected to said supply conduit in parallel flow relationship with saidsecond valve means; second passage means connecting said first fuelpressure differential responsive means to said supply conduit downstreamfrom said second valve means; a restriction in said first passage means;a .Iadd.first .Iaddend.variable area valve in said first passage meansfor controlling the fuel pressure intermediate said restriction and saidvariable area valve; and control means responsive to a plurality ofvariable conditions of engine operation operatively connected to saidvariable area valve for actuating the same; said first fuel pressuredifferential responsive means being .[.verted.]. .Iadd.vented.Iaddend.to said first passage means intermediate said restriction andsaid variable area valve and responsive to the fuel pressuredifferential between said intermediate fuel pressure and said fuelpressure in said second passage means.
 2. Fuel control apparatus asclaimed in claim 1 and further including:a restriction in said secondpassage means; and third passage means having a restriction thereinconnecting said first passage means upstream from said restrictiontherein to said second passage means intermediate said restrictiontherein and said first fuel pressure differential responsive means. 3.Fuel control apparatus as claimed in claim 2 wherein:said restriction insaid second passage means and said restriction in said third passagemeans are fixed and have equal flow areas.
 4. Fuel control apparatus asclaimed in claim 1 wherein:said pressurized source is a positivedisplacement fuel pump; said first valve means includes a .Iadd.second.Iaddend.variable area valve for controlling the effective flow area ofsaid supply conduit and a fuel bypass valve operatively connected tosaid supply conduit for maintaining a constant predetermined fuelpressure differential across said variable area valve.
 5. Fuel controlapparatus as claimed in claim 1 wherein:said variable conditions ofengine operation includes position of a power lever and engine speed. 6.Fuel control apparatus as claimed in claim 5 wherein:said variableconditions of operation further include an engine operating air pressurerelated to engine power output.
 7. Fuel control apparatus as claimed inclaim 6 wherein:said variable conditions of engine operation furtherinclude the temperature and pressure of air consumed by the engine. 8.Fuel control apparatus as claimed in claim 1 wherein:said control meansincludes an electrically actuated proportional solenoid connected tosaid variable area valve and responsive to an electrical input signal;and electronic means responsive to said plurality of variable conditionsof engine operation for .[.producting.]. .Iadd.producing .Iaddend.saidelectrical input signal as a function of said variable conditions ofengine operation.
 9. Fuel control apparatus as claimed in claim 1 andfurther including:position signal generating means operatively connectedto said first positionable valve means; and means operatively connectingsaid position signal generating means and said control means fortransmitting said position signal to said control means.
 10. Fuelcontrol apparatus as claimed in claim 1 and further including:first andsecond stop means operatively connected to said first positionable valvemeans for limiting movement of said first valve means in a closing andopening direction, respectively.
 11. Fuel control apparatus as claimedin claim 8 and further including:an emergency fuel supply conduitconnected at one end to said supply conduit upstream from said firstvalve means and at the opposite end to said supply conduit downstreamfrom said second valve means; normally closed third valve means in saidemergency fuel supply conduit blocking fuel flow therethrough; fourthvalve means in said emergency fuel supply conduit for controlling theeffective flow area thereof; electrically actuated means operativelyconnected to said third valve means for actuating the same to an openposition in response to an electrical power failure; and meansresponsive to a plurality of variable conditions of engine operationoperatively connected to said fourth valve means for actuating the same.12. Fuel control apparatus as claimed in claim 10 wherein:said lastnamed means includes a three dimensional cam contoured as a function ofsaid plurality of variable conditions associated therewith whichincludes position of a power lever and atmospheric air pressure.